I have my backpack bought extra because I have flown with Ryanair and just wanted to take hand luggage. The backpack is exactly the hand luggage dimensions and there were also no problems in controlling, although I have used on the flight back every centimeter of the interior and the backpack looked quite plump. It fits into incredibly much, it is comfortable to wear (through the padding at the back and on the straps) and has many useful smaller compartments. I would recommend this backpack full, happy flight! :)
